Many of the metal sheets used for home appliances, building materials and automobiles are precoated for the purpose of enhancing the design properties and corrosion resistance. For such a metal sheet, a chemical conversion treatment, called a chromate treatment, is applied as a pretreatment to coating in many cases, because the chromate-treated film exhibits excellent corrosion resistance owing to the self-repairing function of hexavalent chromium contained in the film and excellent coating material adhesion owing to the hydrous oxide containing hexavalent chromium.
However, with recent increase in the concern for the global environment, it is required to suppress the elution of hexavalent chromium and, if possible, not to use the chromate treatment.
Against such a background, a technique called resin chromate obtained by compounding an organic resin and chromate has been reported in JP 5-230666 A. By this technique, the elution of hexavalent chromium can be decreased but cannot be completely prevented.
On the other hand, various chromate-free treatments having a performance comparable to the chromate treatment have been recently developed. A representative example thereof is a technique of covering the metal surface with an organic resin having a chelate-forming ability to intensify the bonding force between the cover film and the metal surface. For example, JP 11-29724 A discloses a chromate-free treatment using an aqueous resin containing a thiocarbonyl group-containing compound and a phosphate ion and further containing a water-dispersible silica. However, the coating material adhesion is not necessarily satisfied in uses where severe working is applied.
As described above, a chromate-free treatment with a coating material adhesion at a high level has not been developed and its urgent development is being demanded.